The present invention relates generally to bumped semiconductor components, and more particularly to bumped semiconductor components equipped with redistribution circuitry and suitable for wafer level BGA packaging.
Semiconductor components such as die, chip scale packages, ball grid arrays (BGAs), and wafers frequently include terminal contacts in the form of metal bumps. Components equipped with such contacts are often referred to as xe2x80x9cbumpedxe2x80x9d components.
FIG. 1 illustrates one type of prior art flip chip semiconductor package. The package 10 comprises a semiconductor die 12 and an array of bumped contacts 14 on the circuit side of the die 12. The bumped contacts 14 allow the package 10 to be surface mounted to a substrate, such as a printed circuit board (PCB). Typically, the bumped contacts 14 are made of solder, which allows the package 10 to be bonded to a substrate using a solder reflow process.
The die 12 contained in the package 10 includes a series of contact pads 20 which are in electrical communication with the bumped contacts 14. The die 12 also includes internal conductors 22 which are in electrical communication with the contact pads 20, and with various semiconductor devices and integrated circuits as may be formed on or in the die 12. The die 12 also contains first 16, second 24 and third 38 passivation layers. Typically, the first passivation layer is a material such as plasma oxynitride (PON), and the second and third passivation layers are benzocyclobutene (BCB). One or more openings 26 may be provided through passivation layers 24 and 16 to allow a redistribution conductor 36 (discussed in greater detail below) to be in physical contact with the contact pads 20.
For the sake of clarity, it is to be noted here that the PON layer is typically deposited as two separate layers, one of plasma oxide and the other of plasma nitride. It is treated as a single passivation layer here because deposition of the second layer does not require any intervening processing steps. By contrast, the deposition of each of the BCB layers requires intervening photo steps; hence, these layers are treated as distinct layers, even though their chemical composition may be similar or even identical.
The redistribution conductor 36 is formed on a surface of the second passivation layer 24. The redistribution conductor 36 is sputtered to a thickness typically less than 1 xcexcm and is in electrical communication with the contact pads 20 and the bump contacts 14. The third passivation layer 38 covers the redistribution conductor 36. The redistribution conductor may be used, for example, to redistribute the signals from standard wire contact pads 20 located at the die perimeter to pads of an area array, such as a ball grid array (BGA). As shown in FIG. 1, the redistribution conductor 36 typically requires an under bump metallization (UBM) 44 for each bumped contact 14 to facilitate bonding of the bumped contact 14 to the redistribution conductor 36.
In semiconductor devices in which a flip-chip die is attached to a PCB or other substrate, a substantial amount of stress exists through the entire joint connecting the die to the substrate. This stress arises in part from coefficient of thermal expansion (CTE) differentials between the die and the substrate, with the result that varying amounts of stress and strain are applied to the joint regions as the die and substrate are exposed to thermal cycling. Over time, these stresses can result in mechanical and/or electrical failure of the joint. Accordingly, it has become a common practice in some flip-chip applications to provide an additional underfill material between the third passivation layer 38 and the substrate. This additional underfill material, which typically has a CTE coefficient somewhere between the CTE coefficients of the third passivation layer and substrate, buffers the large CTE differential stress between the third passivation layer and the substrate, thereby reducing or eliminating solder fatigue failure.
In a device such as that shown in FIG. 1, the second and third passivation layers are rigid and serve to mechanically reinforce the redistribution conductors and to clamp them in place. Consequently, a substantial amount of the CTE differential stresses in devices of this type are borne by the solder contacts 14 and by the second 24 and third 38 passivation layers. Indeed, in devices of this type, the redistribution conductors are typically too thin to withstand any significant amount of stress by themselves and tend to break if exposed to significant stresses, thus resulting in electrical failures. While the use of three passivation layers is advantageous insofar as it mechanically reinforces the redistribution conductors, it also has some drawbacks. For example, the addition of a third passivation layer increases the complexity and manufacturing cost of the device, while also making it more difficult to rework the device or to perform electrical probing on the redistribution conductor.
There is thus a need in the art for a die equipped with a redistribution conductor which is suitable for flip-chip applications and which does not require a third passivation layer or an underfill material. There is also a need in the art for a die fitted with a redistribution conductor that can relieve differential CTE stresses. These and other needs are met by the methodologies and devices disclosed herein and hereinafter described.
In one aspect, a device is provided which comprises (a) a semiconductor die or other substrate having a contact pad thereon, (b) a redistribution conductor having a base portion which is in electrical communication with the contact pad, and having a convoluted, laterally extending portion, and (c) a (typically organic) passivation layer disposed between the laterally extending portion and the die, and wherein the laterally extending portion preferably forms a frangible bond to the passivation layer. The device may also comprise a bumped contact in electrical communication with the redistribution conductor. The laterally extending portion may be serpentine or shaped like a sine wave, and preferably changes direction at least once, more preferably at least twice, and most preferably at least three times in going from the base to the bumped contact. The laterally extending portion has an average minimum thickness of at least about 3 microns, preferably within the range of about 8 to about 16 microns, and more preferably within the range of about 10 to about 14 microns, as measured along an axis extending through the center of, and orthogonal to, the laterally extending portion. The device may further comprise a PCB substrate in contact with said bumped contact, in which case the PCB substrate is preferably separated from the redistribution conductor by an open space rather than an underfill. The device preferably also comprises a dewetting agent disposed on surfaces of the laterally extending portion of the redistribution conductor. This dewetting agent, which is preferably sufficiently conductive so as to permit probing of the conductor and is typically about 200 nm in thickness, serves to prevent solder from wetting the redistribution conductor beyond the bump contact region, hence confining the solder bump to an area directly above the bump pad. The dewetting agent can be, but is not limited to, one or more of the materials (e.g., TiW) used as a seed metal for electroplating the redistribution conductor metal.
In another aspect, a device is provided which comprises (a) a semiconductor substrate (which may be, for example, a wafer or die) having a contact pad, (b) a passivation layer, (c) a redistribution conductor having a base portion which is in electrical communication with the die contact, and a laterally extending portion which extends over the passivation layer, and (d) a release layer disposed between the passivation layer and the laterally extending portion. The device may further comprise a bumped contact directly in electrical and mechanical contact with the redistribution conductor at a contact pad (in some embodiments, the bumped contact may instead be in contact with a UBM which is in electrical and mechanical contact with the redistribution conductor at the bump contact pad). The passivation layer is preferably disposed between the laterally extending portion of the redistribution conductor and the semiconductor substrate. The mechanical strength of the metal bump and the metallic redistribution conductor is greater than the adhesion of the metallic redistribution conductor to the passivation layer, which is typically polyimide or BCB. Consequently, although the adhesion between the passivation layer and the metallic redistributing conductor is sufficient to maintain integrity during wafer processing, probing, dicing and through assembly, the redistribution conductor will separate from the underlying passivation layer under sufficiently high stress. Without any outer passivation layer to clamp down the redistribution conductor, the redistribution conductor is able to move sufficiently to relieve stress from the substrate. The laterally extending portion of the redistribution conductor forms a frangible bond to the passivation layer and preferably terminates at one end on a contact pad base. The width of the laterally extending portion is equal to or greater than its thickness to provide mechanical strength, while being narrow enough to separate without line breakage. The laterally extending portion is connected to a bump pad of the same material, and preferably of the same thickness, as the redistribution conductor which forms a site for the solder bump to attach to the redistribution conductor. After the redistribution conductor separates from the underlying passivation layer, its serpentine coil design allows it to stretch and compress to accommodate movement of the solder bump and bump pad.
In yet another aspect, a method for making a semiconductor device is provided which comprises the steps of (a) providing a semiconductor substrate having a contact pad, (b) forming a passivation layer (and preferably two passivation layers, such as a first passivation layer comprising oxide/nitride and a second passivation layer comprising polyimide or BCB) over the substrate and patterning the passivation layer such that at least a portion of the contact pad is exposed, and (c) forming a redistribution conductor having a base portion which is in electrical communication with the contact pad and having a convoluted, laterally extending portion which extends over the passivation layer, wherein the laterally extending portion forms a frangible bond to the passivation layer. The method may further comprise the step of forming a release layer over the passivation layer, and the release layer may be disposed between the redistribution conductor and the passivation layer. The release layer may comprise TiW or other suitable materials which allow the redistribution conductor to separate from the passivation layer under sufficient stress so as to relieve strain on a solder joint connected to the redistribution conductor. The redistribution conductor is preferably formed by depositing a metallization layer over the first passivation layer, depositing and patterning a second passivation layer over the metallization layer, and electroplating the material of the redistribution conductor onto the exposed portion of the metallization layer. The material of the redistribution conductor is preferably electroplated to a minimum thickness of at least about 3 microns as measured along an axis extending through the center of, and orthogonal to, the laterally extending portion.
In still another aspect, a method for making a semiconductor device is provided. In accordance with the method, a semiconductor substrate is provided which has a contact pad. A first passivation layer is deposited over the substrate and is patterned such that at least a portion of the contact pad is exposed, and a metallization layer is deposited over the first passivation layer. A second passivation layer is then deposited over the metallization layer and is patterned such that at least a portion of the metallization layer in the vicinity of the contact pad is exposed. A redistribution conductor is then electroplated onto the exposed portion of the metallization layer such that the redistribution conductor has a base portion which is in electrical communication with the contact pad, and a laterally extending portion. The laterally extending portion preferably has an average minimum thickness of at least about 3 microns as measured along an axis extending through the center of, and orthogonal to, the laterally extending portion. More preferably, the laterally extending portion has an average minimum thickness within the range of about 8 to about 16 microns, and more preferably within the range of about 10 to about 14 microns, as measured along an axis extending through the center of, and orthogonal to, the laterally extending portion. The laterally extending portion is preferably patterned as convoluted or serpentine in shape. The laterally extending portion is typically connected to a solder joint and is preferably adapted to reduce stress applied to the solder joint by delaminating from the second passivation layer. This may be accomplished, for example, by making the bond between the second passivation layer and the metallization layer sufficiently frangible such that the laterally extending portion separates from the second passivation layer when sufficient stress is applied to the solder joint. Preferably, the redistribution conductor comprises copper, the metallization layer comprises a first layer of TiW and a second layer of copper, and the second passivation layer comprises a polyimide. The redistribution conductor is preferably in contact with at least one solder joint, in which case a dewetting agent for the solder may be deposited over portions of the redistribution conductor that are not in contact with the at least one solder joint. Both the metallization layer and the dewetting agent preferably comprise TiW.
These and other aspects are described in further detail below.